1. Field of the Invention
The present invention relates to a semiconductor device as an electrically programmable fuse element to be applied to, e.g., a redundancy circuit of a semiconductor memory, and a method of programming the same.
2. Description of the Related Art
A laser fuse element is conventionally used as a fuse element in a redundancy circuit of a semiconductor memory. Unfortunately, a fuse blow step using a laser system prolongs the TAT (Turn Around Time), and this increases the test cost and deteriorates the ease of the test. To solve these problems, an electrically programmable fuse element (to be referred to as an e-fuse hereinafter) is developed. The e-fuses are classified into various types such as a type by which a gate oxide film is destroyed, and a type by which a line itself is fused by a current stress. In the following description, an e-fuse using a gate wiring structure will be explained.
This e-fuse has two terminals and a fuse link connected between them. The two terminals and fuse link are formed by a CMOS process by using the same material as a gate line. That is, the two terminals and fuse link are made up of, e.g., a polysilicon layer and a silicide layer formed on the polysilicon layer in self-alignment with it. The two terminals are connected to an upper metal layer via contacts and are electrically controllable. The e-fuse is programmed by applying a current stress to the fuse link via the contacts. This programming increases the resistance of the e-fuse.
Examples of this e-fuse programming method are methods using physical phenomena called self-agglomeration (self-assemble) and electromigration.
In the method using self-agglomeration, Joule heat is generated by supplying an electric current to a fuse element. When this heat makes the temperature of the fuse element higher than the temperature of salicide formation, a salicide layer on a polysilicon layer causes self-agglomeration. The self-agglomeration is a phenomenon in which a metal element agglomerates in, e.g., the triple point of the boundary of the polysilicon crystal. When this self-agglomeration occurs, a plurality of regions in which no salicide layer is present on the polysilicon layer are formed. As a consequence, the resistance of the fuse element increases (e.g., reference 1 “A PROM Element Based on Salicide Agglomeration of Poly Fuses in a CMOS Logic Process” IEDM 97, 855-858).
On the other hand, in the method using electromigration, a voltage of, e.g., 3.3 V is applied to a fuse element. Joule heat generated by this voltage raises the temperature of the fuse element to a temperature at which electromigration occurs. At this temperature, a metal element forming a salicide layer on a polysilicon layer is localized to the anode terminal by the electromigration. Also, an impurity (dopant) in the polysilicon layer is localized to the anode by the electromigration. This forms a region not doped with the impurity but made of polysilicon alone in the fuse element. Consequently, the resistance of the fuse element increases (e.g., reference 2 “Electrically Programmable Fuse (eFUSE) Using Electromigration in Silicides” IEEE Electron Device Letters, Vol. 23, No. 9, September 2002).
According to reference 2, in the fuse element using electromigration described above, an electric current of, e.g., 7 mA must be supplied for 200 μs in order to localize the metal element and the impurity element in the polysilicon layer to one of the two terminals by electromigration. Generally, it is difficult to simultaneously cut a large number of e-fuses, because programming requires an electric current of the mA order. Normally, therefore, fuse elements are individually programmed. Accordingly, if programming one fuse element requires 200 μs, programming n fuse elements requires n×200 μs. In addition, an electric current of 7 mA must be kept supplied during the programming. This prolongs the fuse element programming time, and also increases the current consumption.